The present invention relates to a contour correction apparatus capable of controlling the amount of preshoot or overshoot added to a video signal according to its high-frequency component and the correlation quantity between its horizontal scanning lines. More particularly, to an apparatus for improving the non-linearity between the level of the contour correction and a video signal's high-frequency component and horizontal line correlation.
The resolution of an image corresponding to the video signal reproduced by a reproducing unit, such as a VTR or a tuner of a television receiver, can be improved by adding preshoot or overshoot to the transitional portion of a video signal (i.e. a portion at which the video signal amplitude changes). This type of improvement is called contour correction and generally relates to the slope of the change in amplitude of a video signal. For example, by adding preshoot or overshoot to the transitional portion of a luminance signal that is changing from a white to a black level, the contrast between black and white is emphasized, thereby improving its resolution.
However, this type of contour correction apparatus adds a constant amount of contour correction to the transitional portion of a video signal, regardless of the picture's content (i.e., regardless of the video signal's frequency or horizontal line correlation). This insensitivity to video signal frequency and line correlation causes the picture quality to deteriorate in certain cases. For example, when using a constant correction value, the picture will appear rough in areas where motion is prevalent or where the picture makeup is very detailed since the contour correction signal becomes excessive in these areas (i.e. dominates the video signal).
An improved contour correction apparatus corrects these problems by varying the amount of contour correction based on the high-frequency component of the video signal and the amount of correlation between the horizontal scanning lines of the video signal. Specifically, the level of the contour correction is increased as the correlation quantity increases (i.e. the picture includes very little motion). Further, the amount of contour correction is increased as the high-frequency component of the video signal decreases (i.e. the picture has very few details). Conversely, as the correction quantity decreases or the video signal high-frequency component increases, the amount of contour correction decreases. Thus, the proper contour correction is set according to the amount of motion and detail in a picture.
However, this contour correction apparatus directly adjusts the contour control voltage based on the magnitude of the high-frequency component and the amount of correlation between horizontal scanning lines in the video signal. In other words, if the amount of correlation increases by 10%, then the contour control voltage was increased by 10%. Similarly, if the magnitude of the high-frequency component decreases by 10%, then the contour control signal was increased by 10%.
However, the contour correction apparatus uses non-linear semiconductors within the picture control unit to perform the actual contour correction. These semiconductors exhibit a non-linear relation between the control voltage that is input to the picture control unit and the actual contour correction applied to the video signal. FIG. 1 illustrates this non-linear relation between the control voltage applied to the picture control unit and the level (in dBs) of contour correction that is actually added to the video signal. Consequently, the actual level of contour correction does not maintain a linear relation to the high-frequency component and correlation quantities of the video signal. This non-linear relation is disadvantageous.
Further, analog devices are unable to compensate for this non-linear relation between the high-frequency/correlation quantities and the level of contour correction. Specifically, these analog devices are unable to perform the needed inverse correction, which would change the non-linear control characteristic into a linear control characteristic.